Harassment law is not a code of workplace manners; it is a specific legal standard that most bad behavior does not meet, and a specific set of employer obligations that most employers do not satisfy. This article explains what harassment actually is under federal law, why "severe or pervasive" is the phrase that decides most cases, and how the Supreme Court built a framework in which an employer's response to a complaint often matters more than the underlying conduct. It walks through the investigation from both sides: what a competent investigation looks like, what makes one indefensible, and what an employee should expect and demand. It covers retaliation, which is now the most commonly filed charge, and explains why it is easier to prove than the underlying claim. Readers finish knowing what conduct is actionable, what a complaint triggers, and what to do in the first week on either side of it.